diff --git a/lib/prawn/fonts/ttf.rb b/lib/prawn/fonts/ttf.rb
index 5e55220f9..779234de8 100644
--- a/lib/prawn/fonts/ttf.rb
+++ b/lib/prawn/fonts/ttf.rb
@@ -393,18 +393,18 @@ def embed_simple_font(reference, font, unicode_mapping)
           XHeight: x_height
         )
 
-        first_char = font.cmap.tables.first.code_map.index { |gid| !gid.zero? }
-        last_char = font.cmap.tables.first.code_map.rindex { |gid| !gid.zero? }
+        first_char, last_char = unicode_mapping.keys.minmax
         hmtx = font.horizontal_metrics
         widths =
-          font.cmap.tables.first.code_map[first_char..last_char].map do |gid|
-            if gid.zero?
+          (first_char..last_char).map do |code|
+            if unicode_mapping.key?(code)
+              gid = font.cmap.tables.first.code_map[code]
+              Integer(hmtx.widths[gid] * scale_factor)
+            else
               # These characters are not in the document so we don't ever use
               # these values but we need to encode them so let's use as little
               # sapce as possible.
               0
-            else
-              Integer(hmtx.widths[gid] * scale_factor)
             end
           end
 
diff --git a/spec/prawn/font_spec.rb b/spec/prawn/font_spec.rb
index a890c907e..ac9da98f4 100644
--- a/spec/prawn/font_spec.rb
+++ b/spec/prawn/font_spec.rb
@@ -670,4 +670,43 @@ def page_should_not_include_font(font)
     font_name = text.font_settings.first[:name].to_s.sub(/\w+\+/, 'subset+')
     expect(font_name).to eq 'subset+DustismoRoman'
   end
+
+  it 'does not change width of unknown glyph' do
+    text_with_string_widths =
+      Class.new(PDF::Inspector::Text) do
+        attr_reader :string_widths
+
+        def initialize(*)
+          super
+          @string_widths = []
+        end
+
+        def show_text(text, kerned = false)
+          super
+          @string_widths << ((@state.current_font.unpack text).reduce(0) do |width, code|
+            width + (@state.current_font.glyph_width code) * @font_settings[-1][:size] / 1000.0
+          end)
+        end
+      end
+
+    pdf =
+      Prawn::Document.new do
+        font_families.update(
+          'DejaVu Sans' => {
+            normal: "#{Prawn::DATADIR}/fonts/DejaVuSans.ttf",
+            bold: "#{Prawn::DATADIR}/fonts/DejaVuSans-Bold.ttf"
+          }
+        )
+
+        # changing option to subset: false fixes issue (albeit using different behavior)
+        font 'DejaVu Sans', subset: true do
+          text '日本語<b>end</b>', inline_format: true
+        end
+      end
+
+    rendered_pdf = pdf.render
+    analyzed_pdf = text_with_string_widths.analyze(rendered_pdf)
+    expect(analyzed_pdf.string_widths.length).to be 2
+    expect(analyzed_pdf.string_widths[0]).to be > 0.0
+  end
 end
diff --git a/spec/prawn_manual_spec.rb b/spec/prawn_manual_spec.rb
index b92c55441..105016409 100644
--- a/spec/prawn_manual_spec.rb
+++ b/spec/prawn_manual_spec.rb
@@ -6,9 +6,9 @@
 MANUAL_HASH =
   case RUBY_ENGINE
   when 'ruby'
-    '8ace5f35f945e5994647cefc2cf7bc369d131e0646d91eb8aeb94e58f72de18d8e7bf82f58fc45406110c4adad239dcbe834059580d29fec2b2a039db67db04c'
+    '7991e4f72e944140840e1c26f0fff331029846eaab148de8483d06491c7808bc4963e8e7376a514e855037f1f1b4197877a31f2df44f511f4f7f5e0ce5df3170'
   when 'jruby'
-    'b77a740d3290192360c4c083018ca61ccc88d42e7c6a152c7bc394d751f5f08d85aec613549f6660713644b00518561cc0f9c947701f01e8c25632c9db81201a'
+    '29b8f8cb00910426805ce226fb47c59d6409683f35f0d2c056a6cf837ba086ca5c763ff89266cfc8e11b1d92af60c9974822b12ad761cdbdf520adb005a98750'
   end
 
 RSpec.describe Prawn do